This invention relates to a mass spectrograph and more particularly to an atmospheric-pressure ionization interface which may be used, for example, for an liquid chromatograph-mass spectrograph (herein abbreviated as LC-MS).
A LC-MS makes use of an interface (herein referred to as LC-MS interface) for ionizing the sample liquid separated time-wise and eluting from the column of a liquid chromatograph (LC) and leading it to the mass spectrograph (MS). Such a LC-MS interface includes an ionization apparatus for generating gaseous ions while nebulizing the sample liquid by heating it or by using a high-speed gas flow or a strong electric field.
For the ionization in such a LC-MS, a so-called atmospheric pressure ionization method such as the atmospheric pressure chemical ionization method (APCI) or the electroless spray ionization method (ESI) is commonly used. By the APCI, a nozzle is connected to the end of the column of the LC so as to open into an ionization chamber maintained approximately at the atmospheric pressure, and a needle electrode is set in front of its front end. The sample liquid is heated and nebulized as it passes through the nozzle, and the droplets thus produced are ionized by a chemical reaction with the carrier gas ions (or xe2x80x9cbuffer ionsxe2x80x9d) generated by a corona discharge from the needle electrode. By the ESI, an uneven electric field is generated by applying a high voltage of about several kV to the tip of the nozzle such that the sample liquid is dissociated by this electric field and become torn apart by the Coulomb force and nebulized. The solvent contained in the droplets is evaporated by contacting the ambient air such that gaseous ions are obtained.
In order to improve the efficiency of ion generation of such an apparatus for ionization, it is important, say, in the case of ESI, to appropriately adjust the positional relationship between the nozzle for nebulizing the sample liquid and the inlet of the solvent-removing tube for sucking in the ions generated from the nebulized droplets and transporting them to the next stage, as well as that of the glass capillary at the tip of the nozzle for transporting the sample liquid with respect to the metallic tube containing the capillary for applying a high voltage and nebulization gas tube provided coaxially on its outer circumference. For carrying out such adjustments, it is very convenience to make the interior of the ionization chamber observable from outside. For this reason, it has been known to provide a mass spectrograph with a peep hole with a transparent material such as glass on a selected surface of the ionization chamber such that its interior can be visible from outside. With the ionization chamber thus provided, not only is it convenient for the adjustments of aforementioned positional relationships but it also becomes possible to detect the contamination of the nozzle, whether or not the nebulization is taking place normally from the tip of the nozzle and, in the case of the ESI, whether or not the discharge is taking place at the specified position.
If the LC uses a mobile phase containing a large quantity of solvent such as water which is hard to vaporize, however, the solvent tends to cloud the transparent window by contacting or coming close to its inner surface after being sprayed out of the nozzle and gasified and thereby becoming suddenly cooled to condensate. Such a phenomenon is particularly significant in the case of the APCI wherein the interior of the ionization chamber reaches a very high temperature, making it difficult to observe its interior from outside. Since the distance between the peep-hole and the tip of the nozzle is at least several centimeters while the correction on the position of the tip of the nozzle must usually be carried out in units of millimeters, it is a difficult operation to make such a fine adjustment while observing the tip of the nozzle even if the peep-hole is not clouded. Moreover, the interior of the ionization chamber may be dark, depending on where the apparatus is set, because light from outside may not be able to penetrate the interior sufficiently through the peep-hole.
It is therefore an object of this invention in view of the problem as described above to provide a mass spectrograph of which the tip of the nozzle inside the ionization chamber is easily observable such that adjustments of positional relationships can be improved.
A mass spectrograph embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising not only a peep-hole on a wall of its ionization chamber but also a gas supplying means inside the ionization chamber for blowing a dry gas onto the inner surface of the peep-hole. It is preferable to provide the peep-hole with a pane serving as an image-enlarging lens and to coat the inner surface of the pane with a thin film of polytetrafluoroethylene. It is further preferable to provide an illuminating means such as an LED for illuminating the interior of the ionization chamber such that the interior of the ionization chamber can be clearly visible even when the spectrograph is placed inside a relatively dark room.